The object, which the South Korean military said was no longer visible, was spotted two days after Pyongyang test-fired a ballistic missile — its second missile test in seven days.
It’s not the first time South Korea has reacted to an object sent across its border.
The South responded with machine-gun fire after an unidentified North Korean aircraft crossed the DMZ early last year. It “immediately went towards the North,” a South Korean official said at the time.
North Korea is believed to have a fleet of more than 1,300 fighter jets, as well as a unspecified number of its own unmanned aerial vehicles, according to a 2015 report to Congress from the US Department of Defense.
Two suspected North Korean drones were found in South Korea in 2014, one on the island of Baengnyeong and another in the city of Paju, close to the border.